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Old 29-02-2020, 09:50   #1
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Help Understanding AIS

AIS is a great tool, especially in high traffic areas and shipping lanes. However, I am not understanding why I am observing certain phenomenon. The Coast Guard Rules state that many vessels (mostly commercial vessels) must have AIS broadcasting for a minimum of 15 minutes before they are underway. So why am I seeing the following?

Observations:
1) In a marina/ moorage area, there can be many vessels that just leave their AIS on even while tied up or moored (often for months at a time!). This leads to many false AIS warnings that cause me to turn off my receiver near moorages so that my chart plotter is not bombarded with so many alerts that it can become virtually unusable. Is this just sloppiness on the vessels' captains part or ???

2) Some vessels seem to turn off AIS or stop broadcasting while stationary. The best example are the State Ferries here in Washington. While boarding vehicles, the ferries stop broadcasting AIS, and only start rebroadcasting just as they cast off and leave the dock (while blasting their horns and yelling for all small craft to get out of their way over the VHF). They definitely don't adhere to the 15 minute rule. You can't tell if thy are about to leave or moored at the terminal for an extended period. I spoke with a ferry captain, and he insisted they do not turn AIS off while boarding (contrary to many observations by me).

I would appreciate any enlightenment, please.
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Old 29-02-2020, 10:21   #2
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Re: Help Understanding AIS

Are you sure of receiving well the AIS signal ?2.11.0.0
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Old 29-02-2020, 10:39   #3
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Re: Help Understanding AIS

Yes, I am receiving well.
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Old 29-02-2020, 10:42   #4
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Re: Help Understanding AIS

People leave AIS on so they can look on marine traffic and see their boats. No. I don’t get it either.
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Old 29-02-2020, 10:43   #5
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Re: Help Understanding AIS

Depending upon the Class ("A" or "B") and the movement, the rate of transmission is automatically adjusted, so a docked vessel will transmit less often than when underway. The Wiki page at https://en.wikipedia.org/wiki/Automa...ication_system give a lot of details and should answer your question.
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Old 29-02-2020, 10:55   #6
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Re: Help Understanding AIS

Quote:
Originally Posted by Zanshin View Post
Depending upon the Class ("A" or "B") and the movement, the rate of transmission is automatically adjusted, so a docked vessel will transmit less often than when underway. The Wiki page at https://en.wikipedia.org/wiki/Automa...ication_system give a lot of details and should answer your question.
As per wiki linked above.

An AIS transceiver sends the following data every 2 to 10 seconds depending on a vessel's speed while underway, and every 3 minutes while a vessel is at anchor:

The vessel's Maritime Mobile Service Identity (MMSI) – a unique nine digit identification number.
Navigation status – "at anchor", "under way using engine(s)", "not under command", etc.
Rate of turn – right or left, from 0 to 720 degrees per minute
Speed over ground – 0.1-knot (0.19 km/h) resolution from 0 to 102 knots (189 km/h)
Positional accuracy:
Longitude – to 0.0001 minutes
Latitude – to 0.0001 minutes
Course over ground – relative to true north to 0.1°
True heading – 0 to 359 degrees (for example from a gyro compass)
True bearing at own position. 0 to 359 degrees
UTC Seconds – The seconds field of the UTC time when these data were generated. A complete timestamp is not present.

In addition, the following data are broadcast every 6 minutes:

IMO ship identification number – a seven digit number that remains unchanged upon transfer of the ship's registration to another country
Radio call sign – international radio call sign, up to seven characters, assigned to the vessel by its country of registry
Name – 20 characters to represent the name of the vessel
Type of ship/cargo
Dimensions of ship – to nearest meter
Location of positioning system's (e.g., GPS) antenna on board the vessel - in meters aft of bow and meters port or starboard
Type of positioning system – such as GPS, DGPS or LORAN-C.
Draught of ship – 0.1 meter to 25.5 meters
Destination – max. 20 characters
ETA (estimated time of arrival) at destination – UTC month/date hour:minute
optional : high precision time request, a vessel can request other vessels provide a high precision UTC time and datestamp
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Old 29-02-2020, 11:04   #7
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Re: Help Understanding AIS

People leaving AIS on in marinas is my pet peeve
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Old 29-02-2020, 11:05   #8
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Re: Help Understanding AIS

Quote:
Originally Posted by Captain Puget View Post
AIS is a great tool, especially in high traffic areas and shipping lanes. However, I am not understanding why I am observing certain phenomenon. The Coast Guard Rules state that many vessels (mostly commercial vessels) must have AIS broadcasting for a minimum of 15 minutes before they are underway. So why am I seeing the following?

Observations:
1) In a marina/ moorage area, there can be many vessels that just leave their AIS on even while tied up or moored (often for months at a time!). This leads to many false AIS warnings that cause me to turn off my receiver near moorages so that my chart plotter is not bombarded with so many alerts that it can become virtually unusable. Is this just sloppiness on the vessels' captains part or ???

2) Some vessels seem to turn off AIS or stop broadcasting while stationary. The best example are the State Ferries here in Washington. While boarding vehicles, the ferries stop broadcasting AIS, and only start rebroadcasting just as they cast off and leave the dock (while blasting their horns and yelling for all small craft to get out of their way over the VHF). They definitely don't adhere to the 15 minute rule. You can't tell if thy are about to leave or moored at the terminal for an extended period. I spoke with a ferry captain, and he insisted they do not turn AIS off while boarding (contrary to many observations by me).

I would appreciate any enlightenment, please.


If a boat is moored / tied in a marina and not planning to depart within 15 minutes then it should not be broadcasting. If it is moored in a moorage field, [or at anchor on a temporary mooring, i.e., dropped anchor] then it should always be broadcasting.

Per answers to FAQs at the USCG:

https://www.navcen.uscg.gov/?pageName=AISFAQ#18

When must AIS be in operation? Per 33 CFR 164.46(d), vessels required to have AIS must operate it in U.S. navigable waters (as defined in 33 CFR 2.36) at all times that the vessel is navigating (underway or at anchor) and at least 15 minutes prior to unmooring. Should continual operation of AIS compromise the safety or security of the vessel or where a security incident is imminent, the AIS may be switched off. This action and the reason for taking it must be reported to the nearest U.S. Captain of the Port or Vessel Traffic Center and recorded in the ship's logbook. The AIS should return to continuous operation as soon as the source of danger has been mitigated. Note, vessels equipped with AIS--either by mandatory carriage or voluntarily--must abide by the requirements set forth in 33 CFR 164.46 which state an AIS must be: properly installed, use an officially assigned MMSI, that its data be accessible from the primary operating position of the vessel, and, always be in effective operating condition; which entails the continuous operation of AIS and the accurate input (see USCG AIS Encoding Guide) and upkeep of all AIS data parameters. Although Coast Guard AIS authority (46 USC 70114) does not extend beyond U.S. navigable waters or to all voluntary users, mariners are reminded that Navigation Rule 7 requires that every vessel use all available means to determine risk of collision. AIS is one of the most effective means currently available, particularly when coupled with radar and sight, to not only determine the risk of, but, also mitigate collisions. Thus the Coast Guard exhorts all AIS users to maintain their AIS in effective operation, at all times.
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Old 29-02-2020, 14:39   #9
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Re: Help Understanding AIS

Quote:
Originally Posted by Wayne hoath View Post
People leave AIS on so they can look on marine traffic and see their boats. No. I don’t get it either.
They are also left on to alert an owner to unauthorized movement of their vessel. The pro who installed my unit told me he has installed “hidden” units in several go fast powerboats and they have allowed for quick apprehension of waterborne thieves.
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Old 29-02-2020, 16:50   #10
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Re: Help Understanding AIS

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They are also left on to alert an owner to unauthorized movement of their vessel. The pro who installed my unit told me he has installed “hidden” units in several go fast powerboats and they have allowed for quick apprehension of waterborne thieves.
Very clever. So basically the AIS becomes like a vessel's Lojack, except it does not need to receive a signal to activate its location beacon. So the maritime authorities are issued a BOLO and scan the AIS data to find and track the vessel.
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Old 29-02-2020, 17:43   #11
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Re: Help Understanding AIS

I actually wired mine up so it comes on when either engine "ignition" key is on. I can override it, but that's the default behavior. Seems to work pretty well. I had some notion that it would also serve as sort of a locator beacon in the event of theft but, realistically, nobody's going to steal a boat as slow as mine.

Still, I'm surprised this isn't the norm, at least on power boats.
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Old 29-02-2020, 18:02   #12
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Re: Help Understanding AIS

On your #1 issue:
If you don't want 'false' alarms then change the alarm criteria on your receiver. Some units, like my Vesper, have alarm profiles that you can setup. For example one for offshore and one for harbor. The criteria to alarm is very different between the two. Other systems offer the possibility to only alarm on vessels moving faster than a set criteria. Alarms are the receivers responsibility.
There is no requirement to turn off your AIS when moored.

On #2.
As noted above the transmission rate is based on vessel speed. The AIS system is not a guranteed delivery system. The packet is broadcast, your system may or maynot receive it. If you drop one or two packets then the wait is more than doubled to see the next update.
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Old 29-02-2020, 18:12   #13
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Re: Help Understanding AIS

As I understand it the rule is that you must turn it on "at least" 15 minutes before leaving. There's no rule against just leaving it on. Most commercial ships seem now to just leave it on.

I also have a Vesper so I turn off the alarm for any vessel moving less than 2 knots. Turning off the alarm doesn't make it disappear from the screen.

Except when offshore, I rarely have any audible alarms on - they drive me nuts in any congested waters. I'm monitoring the chart plotter closely enough for regular navigation that I see the AIS target (along with its course and CPA) in plenty of time.

Finally, my Vesper has a great anchor alarm that I prefer to the ones on my phone/Ipad or the chart plotter . So when anchored, I always leave it on for the anchor alarm and to be more visible to anyone coming through the anchorage.
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Old 01-03-2020, 08:08   #14
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Re: Help Understanding AIS

Quote:
Originally Posted by CaptTom View Post
I actually wired mine up so it comes on when either engine "ignition" key is on. I can override it, but that's the default behavior. Seems to work pretty well. I had some notion that it would also serve as sort of a locator beacon in the event of theft but, realistically, nobody's going to steal a boat as slow as mine.

Still, I'm surprised this isn't the norm, at least on power boats.
Sounds like a viable scheme so that one does not forget to activate it when you go motoring about, but you would need to remember to override it when at anchor or mooring or 15 minutes before going underway from a marina / wharf.

I think that quite a few boaters simply forget to turn it off once they get settled in a marina and are under shore power such that the unit remains transmitting when they go home for the week or month.
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Old 01-03-2020, 17:37   #15
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Re: Help Understanding AIS

Quote:
Originally Posted by CaptTom View Post
I actually wired mine up so it comes on when either engine "ignition" key is on. I can override it, but that's the default behavior. Seems to work pretty well. I had some notion that it would also serve as sort of a locator beacon in the event of theft but, realistically, nobody's going to steal a boat as slow as mine.

Still, I'm surprised this isn't the norm, at least on power boats.
Not sure why this would be a good norm. How about when anchored in fog or poor visibility, or any emergency when the engines are off and you are in a channel?
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